1. Field of the Invention
The present invention relates to an optical element and to an optical system used in a variety of optical devices, such as video cameras, still cameras, portable device cameras, and other image pickup apparatuses, and head mounted displays, projectors, and other display devices.
2. Related Background Art
Design methods and design examples for sufficiently correcting aberrations in a decentered optical system by introducing an idea referred to as a reference axis, and making a constituent surface of the optical system into an asymmetric non-spherical surface, are disclosed in U.S. Pat. No. 5,825,560, U.S. Pat. No. 5,847,887, U.S. Pat. No. 6,021,004, U.S. Pat. No. 6,166,866, U.S. Pat. No. 6,292,309, U.S. Pat. No. 6,366,411, and the like.
The decentered optical system is referred to as an off axial optical system. Consider the reference axis across a light ray that passes through the center of an image and the center of a pupil. The decentered optical system is defined as an optical system that contains a curved surface (off axial curved surface) where a surface normal at an intersection between the reference axis and the constituent surface is not on the reference axis. The reference axis takes on a bent shape in the off axial optical system.
The constituent surface generally becomes decentered in the off axial optical system, and vignetting does not develop even in a reflecting surface. It is thus easy to build the optical system that uses the reflecting surface. Further, the optical path can be relatively freely drawn around, and it is easy to make an integrated optical system by using the method of integrally forming the constituent surface. Accordingly, reflecting optical elements that are compact, freely shapeable, and that utilize space efficiently can be configured.
Further, U.S. Pat. No. 6,268,963 and U.S. Pat. No. 2003-063400 disclose compactly structured optical prisms that secure a sufficient optical path length by making the optical path. (reference axis) cross itself in an inner portion of the optical prism.
However, for cases where the number of reflecting surfaces of the optical prism increases due to a reason such as correcting aberrations in the optical prism, the influence of errors during manufacture of each reflecting surface, such as surface shape errors or surface peculiarities, accumulates. The tolerances in error in each of the reflecting surfaces become smaller and more severe as the number of reflecting surfaces increases. Accordingly, high precision must be secured for the surface shape of each of the reflective surfaces.
The manufacture of such optical prisms has been carried out using metallic molds in recent years mainly due to the demand for lower manufacturing costs. Further, it is necessary to provide a projecting portion to both sides of an optical element, and hold the optical element by a holding member using the projecting portions or the like when the optical element is attached to an actual optical device. However, the projecting portions are generally formed in split portions of a mold that has been split into two pieces.
FIG. 8 shows a reflecting optical element P disclosed in U.S. Pat. No. 2003-063400. Further, FIG. 9 shows optical surfaces R11 to R15 of the optical element P of FIG. 8, a center principal light ray (reference axis) PR, and normals RV1 to RV5 to the optical surfaces R11 to R15, respectively. The normals RV1 to RV5 of the optical surfaces R11 to R15 extend in mutually different directions.
FIG. 10 is a schematic diagram that shows a state where the optical element P of FIG. 8 is formed by a metallic mold that is split into two pieces. In FIG. 10, the metallic mold is split in the Y-direction shown in FIG. 8, and the mold is opened in the Y direction after the optical element P is formed. Symbol UM here denotes an upper mold, and symbol DM denotes a lower mold.
Referring to FIG. 11, mold surfaces RM11 and RM12, which correspond to the optical surfaces R11 and R12, respectively, are either substantially parallel to, or inclined at a steep angle with respect to, a direction in which the mold opens (the Y direction). The optical element P consequently tends to deform during removal from the mold.
Further, FIG. 12 shows a state when the mold surfaces RM11 and RM12 are machined when manufacturing the upper mold UM. Symbol B here denotes a cutting tool that is attached to a cutting machine. The cutting tool B is normally brought into contact with mold surfaces from a direction normal to the surfaces during manufacture of the mold. However, as can be understood from FIG. 12, a position of the cutting tool B when cutting the wall RM11 differs from a position of the cutting tool B when cutting the surface RM12 by an angle of nearly 180° C. However, normal cutting machines cannot handle such a large change in the position of the cutting tool. Rather, it is only possible for the cutting machines to change the cutting tool position by an angle on the order of 80° to 120° at most. Further, the smaller the gap between the surface RM11 and the surface RM12, the more difficult it becomes for the cutting tool B itself to enter between the surface RM11 and the surface RM12. Accordingly, it becomes impossible for the cutting tool B to perform cutting.
Considering the deformation upon removal from the mold, the machining of the mold, and the like, it is preferable that the angle between the normals of the optical surfaces and the directions in which the mold is removed be small.
Problems such as these are more or less resolved by splitting the mold into three or four pieces instead of two. However, it becomes more difficult to maintain positional precision of the mold pieces when the mold is split into three or four pieces, and the mold is thus not suited for forming a reflective surface for which higher precision is required than that of a light transmitting surface. In addition, the structure of a molding apparatus becomes more complex compared to that used when the mold is split into two pieces. This leads to higher costs.
In other words, the parting plane of the mold must be designed carefully in order to manufacture the optical element such as that-described above in which the reference axis crosses itself in the inner portion of the optical element. The same applies to the optical element disclosed in U.S. Pat. No. 6,268,963.